Selenium is a chemical element with the symbol Se and atomic number 34. It is a nonmetal with several important properties and applications.

Key Characteristics of Selenium:

1. Physical Properties: Selenium has a grayish appearance and exists in several allotropic forms, including amorphous and crystalline structures. It is a brittle solid with a relatively low melting point of 221°C (430°F) and a boiling point of 685°C (1,265°F). Selenium exhibits photovoltaic properties, meaning it can convert light energy into electricity.
2. Chemical Properties: Selenium is chemically reactive and can form compounds with various oxidation states, including -2, +4, and +6. It has good electrical conductivity in certain forms and behaves as both a semiconductor and photoconductor.
3. Abundance and Occurrence: Selenium is not as abundant as some other elements, occurring at an average concentration of about 0.05 parts per million in the Earth’s crust. It is primarily found in combination with other elements, such as sulfur, in minerals like selenite, selenide, and selenate. Selenium is also present in coal, soils, and water.
4. Photovoltaic Properties: Selenium has been used in the production of photovoltaic cells, also known as solar cells. It can convert light energy into electricity when exposed to light, making it suitable for certain solar energy applications. However, selenium’s use in photovoltaics has been largely replaced by other materials.

Applications of Selenium:

1. Glass Manufacturing: Selenium compounds are used in the production of glass to impart a red color. Selenium dioxide can be added to glass compositions to achieve a red tint, which is desirable for decorative purposes and in specialty glass applications.
2. Photocopiers and Printers: Selenium has been used in older photocopiers and laser printers as a photoconductor. It can convert light energy into an electrical charge, enabling the formation of electrostatic images that are transferred to paper during the printing process.
3. Metallurgy and Alloys: Selenium can be used as an alloying element in certain types of stainless steel and nonferrous alloys. It improves the machinability and corrosion resistance of these alloys.
4. Dietary Supplements and Nutrition: Selenium is an essential micronutrient for humans and animals. It is required in small amounts for various biological processes, including antioxidant defense systems. Selenium supplements are sometimes used to address selenium deficiency, particularly in areas where soil selenium levels are low.
5. Pharmaceuticals: Selenium compounds have been investigated for their potential therapeutic applications. Some selenium compounds exhibit antioxidant properties and have been studied for their potential role in preventing certain diseases and supporting overall health.
6. Environmental Remediation: Selenium can be used in environmental remediation efforts to treat contaminated sites. Certain forms of selenium, such as selenium oxyanions, can be used to remove heavy metals from water and soil through precipitation or adsorption processes.

It’s important to note that while selenium is an essential nutrient, excessive intake can be toxic. Selenium poisoning, known as selenosis, can occur if high levels of selenium are ingested or inhaled. Care should be taken to ensure appropriate levels of selenium intake and avoid excessive exposure.

In summary, selenium’s applications in glass manufacturing, photocopiers, alloys, dietary supplements, and potential pharmaceutical uses highlight its diverse range of uses. While its use in photovoltaics has decreased, selenium’s unique properties and essentiality in trace amounts make it relevant in various industries and biological systems.